diff --git a/slirp/tcp_subr.c b/slirp/tcp_subr.c
index dc8b4bbb50e..da0d53743f5 100644
--- a/slirp/tcp_subr.c
+++ b/slirp/tcp_subr.c
@@ -148,7 +148,16 @@ tcp_respond(struct tcpcb *tp, struct tcpiphdr *ti, struct mbuf *m,
 		m->m_data += IF_MAXLINKHDR;
 		*mtod(m, struct tcpiphdr *) = *ti;
 		ti = mtod(m, struct tcpiphdr *);
-		memset(&ti->ti, 0, sizeof(ti->ti));
+		switch (af) {
+		case AF_INET:
+		    ti->ti.ti_i4.ih_x1 = 0;
+		    break;
+		case AF_INET6:
+		    ti->ti.ti_i6.ih_x1 = 0;
+		    break;
+		default:
+		    g_assert_not_reached();
+		}
 		flags = TH_ACK;
 	} else {
 		/*